1. Field of the Invention
This invention relates to a method for reducing pulp to powder and thereby producing powder pulp for use, for example, in the production of cellulose ethers. Moreover, this invention also relates to a process for the production of a cellulose ether.
2. Description of the Related Art
In the production of cellulose ethers, highly purified cotton linter pulp and wood pulp are ground by means of a grinder such as a knife mill, and the resulting powdered pulps are used as starting materials.
As methods for grinding pulp finely, there have been proposed a method in which dried pulp is ground at low temperatures as described, for example, in Japanese Patent Provisional Publication No. 59-75901 and Japanese Patent Publication No. 64-7828; and a method in which compressed pulp is ground by means of a jet mill as described, for example, in Japanese Patent Publication No. 3-48010.
However, these methods are not satisfactory from an industrial point of view, because they involve a troublesome step (e.g., drying or compression) prior to grinding and are hence unsuitable for the treatment of large amounts of pulp, and because they require an apparatus for keeping the grinder at a low temperature and a refrigerant therefor.
In order to overcome these disadvantages, grinding in a knife mill is widely employed at present.
Usually, powdered cellulose ethers are used by dissolving them in a solvent such as water. The undissolved fiber content in an aqueous solution of a cellulose ether is considered to be dependent on the degree of uniformity to which an alkali permeates into powdered pulp during the preparation of an alkali cellulose. The undissolved fiber content in an aqueous solution of a cellulose ether may pose a problem during its use. Consequently, in order to reduce the undissolved fiber content, it is essential to cause an alkali to permeate uniformly into pulp and thereby prepare an alkali cellulose having a uniform alkali concentration.
Moreover, the permeation of an alkali into powdered pulp is considered to be affected by the particle shape of the powdered pulp. Grinding in a knife mill utilizing chiefly shearing force yields powdered pulp comprising long fibers when observed microscopically. Since a fibrous powder has a small surface area per particle, this limits the chance of contact between the pulp and the alkali, and acts adversely on the permeation of the alkali into the particles. Consequently, it is thought that the alkali fails to permeate uniformly into the powdered pulp and the degree of alkali permeation thereinto is limited.
Furthermore, in a fibrous powder, the fibers are intertwined with one another to give a large void volume and hence a low bulk density. Since the amount of pulp which can be used at a time in the production of a cellulose ether is limited, fibrous powdered pulp having a low bulk density is disadvantageous from the viewpoint of production. Accordingly, it is desired to develop a method for producing powdered pulp having as high a bulk density as possible on an industrial scale.